A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. A typical RAT in current use is described in U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power, or both in the case of a hybrid RAT.
The general operation of a RAT requires that it rotate from a stored position to a deployment position in an air stream. The rotating portion of the RAT comprises an air stream-powered turbine, lower gearbox and strut. The stationary portion of the RAT comprises a RAT frame and a pivot post that has journal bearings in which the strut rotates. The storage for the RAT is typically within a RAT bay behind a RAT bay door. The RAT must only start rotating after it achieves adequate clearance from the RAT bay door so that its blades may rotate freely. The blades are locked until a release mechanism allows the blades to rotate. The release mechanism usually comprises a turbine release cable coupled to a turbine release cable lever on one end and a turbine release pin on the other. The turbine release pin passes through an aperture in the turbine driveshaft when the RAT is in the stowed position to lock the blades whilst stowed. As the RAT rotates out of the RAT bay, the turbine release lever rotates with it until it hits a stop pin. Since the turbine release lever then remains motionless as the RAT continues to rotate into its deployment position, the turbine release cable to which it attaches pulls the turbine release pin out of the aperture in the turbine driveshaft so that the blades are free to rotate.
When the RAT is ready to be re-stowed into the RAT bay, the blades must align in position so that they can return through the RAT bay door. Misalignment of the blades can cause them to contact the door and damage can result. A stow abort mechanism can prevent such damage by inhibiting RAT stowage when the blades are misaligned.
The common stow abort mechanism generally comprises a turbine release cable sensing system that comprises an electronic switch with an associated electrical wiring harness and stow panel logic circuitry rigged to sense misalignment of the blades based upon the position of the turbine release cable. That is, if the blades are misaligned, the turbine release pin fails to re-insert itself into the turbine drive shaft, thereby forcing the turbine release cable against the turbine release cable lever and thereby displacing the turbine release cable lever away from the stop. This activates the electronic switch and the stow abort mechanism inhibits the stow operation. The problem with this type of stow abort system is that the switch, rigging, electrical harness and stow panel logic circuitry are somewhat costly. A simpler and less expensive approach is desirable.